1. Field of the Invention
This invention relates to a wire mesh filter configuration and its use in particulate filters, especially for soot-generating combustion engines, and to a method for making such filters.
2. State of the Art
Diesel engines are particularly well-adapted for heavy work because of their durable design. However, the burning, or internal combustion, of diesel fuel generates a significant amount of particulate matter, such as soot, and much more so than gasoline engines generate.
Recent apparatus for treating diesel exhaust include a metal or ceramic filter in the exhaust stream to trap the particulate matter, often called a “diesel trap.” Sensors determine the amount of particulate trapped in or on the filter by changes in the resistance or conductivity of the filter, which changes as soot accumulates on the filter element. At a predetermined fouling of the filter (as determined by the electrical resistance or conductivity), the filter is regenerated by heating the filter. Heating is typically accomplished, especially with a metal filter, by running a current through the filter to generate heat by resistance; ceramic and cermet filters can be heated by having a resistance heating element in contact with the filter.
Kawamura (U.S. Pat. No. 5,651,248, U.S. Pat. No. 5,651,250, and U.S. Pat. No. 5,809,777) discloses a particulate filter made from a combination of a sheet filter, formed by sintering a slurry of ceramic fibers, and a wire mesh heater. The sheet filter can be in the form of a tube having pleats running along the length of the tube with wire heaters disposed within the pleates on the outer surface.
Ban et al. (U.S. Pat. No. 6,024,927) describes a cermet filter having an integral heating element. The filter can be in a cylindrical geometry, and one filter can be nested within another.
Bolt (U.S. Pat. No. 5,557,923) discloses a mesh filter for particulate exhausts in the shape of a cylinder, truncated cone, and/or cone, the filter being regenerable.
Buck (U.S. Pat. No. 5,571,298) discloses a regenerable filter made from mineral fibers and metal wires.
Peter et al. (U.S. Pat. No. 6,063,150) discloses a self-cleaning and self-healing particulate filter in the form of a wire mesh filter sandwiched between sintered metal heating elements.
One of the issues not addressed by the prior art is that when a wire mesh is wound into a cylindrical form, the exhaust gas pressure can cause the mesh to extend (“telescope”) out of the spiral/cylindrical form. The use of flanges and other guards to keep the mesh from telescoping can restrict the gas flow more than desired. Especially when the intent of the industry is to use a low density mesh, providing decreased resistance to flow and hence better mileage, an increase in exhaust flow resistance is counterproductive. Accordingly, there is a need for a low density mesh design that can be wound and will not telescope.